The present invention relates to fibre optic data links and in particular to the merging of multiple separate input data streams into a single data stream for transmission. More particularly, the invention relates to a data format using a single word which includes multiple synchronous data streams of different clock rates and multiple isochronous data streams of different clock rates and one or more asynchronous data stream or streams. This single word is split into multiple synchronous data streams for transmission.
Prior art methods of data transmission require multiple wire, high cost cables and suffer from limited transfer distance, degraded Front of screen (FOS) performance and Electro-Magnetic Compatibility (EMC) problems. The majority of computer systems at present utilise CRT monitor display systems and it is the analogue nature of the signals required by these display monitors which is responsible for imposing the distance limits over which the data can be delivered. As LCD flat panel monitors become more prevalent, due to decreasing costs, it is no longer required to deliver the data in an analogue format but to use their native digital format for transmission from the PC system unit to the display. Until recently this has been achieved by a double conversion process, firstly digital-analogue and secondly analogue to digital, which allowed the industry standard analogue interface to be used but suffers the problems referred to above and also further signal degradation from the double conversion process.
Fibre optic data links are well known and have the advantages of good noise immunity and high bandwidth. The current technology of fibre optic data links is generally designed for telecommunications applications in which communications over distances of tens of kilometers is required with a very low error rate. Such links are asynchronous digital links having multiple input data streams, and include, for example, ISDN. The data structures in the fibre optic link are very different to that used by the equipment between which communication is taking place by means of the fibre optic link.
Whilst such known fibre optic links work well for telecommunications applications at, for example, 1.0 Gigabits/sec or at 2.4 Gigabits/sec, the cost of the link is high. In telecommunications applications, this cost is shared by the multiple separate pieces of equipment which are using the fibre optic link to communicate.
The benefits of good noise immunity and high bandwidth mean that the use of fibre optic links for non-telecommunications applications is increasing. Such applications are distinguished from telecommunications applications by virtue of the fact that they rarely exceed 150 meters in length and are frequently as short as 2 meters in length. The cost of a telecommunications type of fibre optic link for such an application is between 10 and 100 times too expensive. The physical size of the equipment for a telecommunications fibre optic data link is too large for easy incorporation into a personal computer, computer display or an input/output sensor. When used as a data link from a personal computer to a computer display, the video data that is sent from the personal computer to the computer display can be permitted to have transmission errors, but the synchronisation (or control) signals cannot be permitted to have transmission errors, otherwise the displayed image will break up and the errors will be visible to the end user.
U.S. Pat. No. 4,863,233 discloses a system for connecting a personal computer to a computer display using three discrete fibre optic cables for each of the Red, Green and Blue video data. Vertical and Horizontal sync signals are added to, for example, the Red and Blue video fibre optic cables. This system is a digital isochronous link having a single input stream.
U.S. Pat. No. 5,132,828 discloses a system for connecting a personal computer to a computer display using a fibre optic cable for each channel of video, the video signals being analog signals. The gain of a video amplifier is compensated to adjust for variations in the gain of the fibre optic link. U.S. Pat. No. 5,132,827 describes a similar system. These systems are analogue video systems.
The information transmitted over a data link can be split into two types of information, asynchronous data and isochronous data. The transmission of isochronous data requires predictable, periodic access to the data link. The transmission of asynchronous data does not require such predictable, periodic access.
European Patent 0 174 099 A describes a system for connecting a computer to a computer display using a fibre optic cable. The fibre optic link is bidirectional carrying unidirectional isochronous video data from the computer to the computer display and asynchronous keyboard control and sound synthesis data from the computer to the computer display and asynchronous keyboard input data, mouse input data and speech input data from the computer display to the computer. The clock frequency for the control data is at the same rate as that for the video data and the control data is in the same format as the video data. The control data is time domain multiplexed with the video data. This system is a digital isochronous link having a single input stream, together with a bidirectional asynchronous link having a multiple input streams.
U.S. Pat. No. 5,450,411 discloses a system in which non-isochronous data streams are merged together with isochronous data streams to form an asynchronous ATM data stream for transmission over an ATM link.
U.S. Pat. No. 5,396,494 discloses a system in which an asynchronous packet transmit bus is provided such that isochronous information packets may be transmitted from a source to a destination with a fixed, constant delay. Arbitration is used between the various transmission sources.
U.S. Pat. No. 5,640,392 discloses a system in which isochronous data is received continuously without interruption and irregular asynchronous data is also received. A single isochronous data stream and a single asynchronous data stream is used. FIFOs are used for storing the isochronous and asynchronous data.
U.S. Pat. No. 5,392,280 discloses a system in which both synchronous transmission and asynchronous transmission are used in an alternating pattern. This is achieved by cell level dynamic slot allocation.
U.S. Pat. No. 5,173,901 discloses a system in which synchronous and asynchronous data streams are transmitted over a single transmission link. The time frames of the input and output cell streams are phase synchronous.
U.S. Pat. No. 5,603,058 discloses a video optimised media streamer having communications nodes receiving digital data from a storage node and transmitting the data to adapters for generating isochronous digital data streams.
It would be advantageous if a combined high speed unidirectional isochronous data link together with an interleaved medium speed bidirectional isochronous data link could be provided. It would also be advantageous if the data rate across the data link could be reduced.
Accordingly, the invention provides an isochronous output data stream format in which a single n bit word is transmitted as m multiple synchronous data streams where the length of the word transmitted in each data stream is n/m, the output data stream containing multiple isochronous different data rate data streams and one or more asynchronous data stream or streams.
Preferably, one bit from each of the m multiple synchronous data streams are combined and transmitted as a single signal at one of 2m analog levels. This reduces the data rate which needs to be sent across the data link by a factor of m.
Preferably, data which is it desired to have a lower error rate than other data is transmitted in the synchronous data stream corresponding to the most significant bit associated with the 2m analog levels. The most significant bit of the data link is less likely to have data errors than the less significant bits as the signal amplitude is larger, so this is used for such information as synchronisation signals, calibration signals and the like.
Preferably, one of the isochronous data streams is update or refresh data to a display transducer or update or refresh data from a sensor or one of the isochronous data streams is isochronous IEEE1394 data or USB data or STM data or RS-442 data.
Further preferably, one or more of the asynchronous data steams is Asynchronous IEEE1394 data or Asynchronous USB data or DDC data or I2C data or ATM data.
In a preferred embodiment, n=32 and m=4.